1. Field of the Invention
The present invention relates to a liquid crystal device, and more specifically, a transmission type active matrix liquid crystal device.
2. Description of Related Art
Liquid crystal devices are energetically researched and developed as display apparatuses and light modulation devices. At present, a direct-viewing type display apparatuses using the liquid crystal devices are widely used, and the liquid crystal devices have been incorporated into projection type display apparatuses and printer heads.
In general, the liquid crystal device is fundamentally formed of a pair of electrode plates opposing to each other, and a liquid crystal material sandwiched between the pair of electrode plates. In a liquid crystal device including a plurality of pixels, the electrodes formed on one of the pair of electrode plates are, in many cases, different in size from those formed on the other electrode plate. Particularly, in a liquid crystal device including switching devices such as a TFT (thin film transistor), the switching devices and pixel electrodes connected thereto are formed on one of the pair of electrode plates (main plate), and a common opposing electrode is formed on the whole of the other electrode plate (opposing plate).
FIG. 1A illustrates, in an enlarged scale, one pixel portion of a main plate of a conventional active matrix liquid crystal display apparatus, and FIG. 1B shows a sectional view taken along the line C--C in FIG. 1A in the same liquid crystal display apparatus. In these figures, Reference Numeral 301 designates a main glass plate, and Reference Numeral 302 shows a scan line. Reference Numeral 303 indicates a signal line. Reference Numeral 305 is given to a switching device such as TFT (thin film transistor). Reference Numeral 306 designates a pixel electrode, and Reference Numeral 308 is an opposing glass plate. Reference Numeral 309 shows an opposing electrode. Reference Numeral 310 indicates an inter-pixel light blocking layer, and Reference Numeral 311 is an aligning film. Reference Numeral 312 designates a liquid crystal layer.
In this liquid crystal display apparatus, the transparent pixel electrode 306 and the transparent opposing electrode 309 are formed on the main plate 301 and the opposing plate 308, respectively, and furthermore, the aligning films 311 are formed on the transparent pixel electrode 306 and the transparent opposing electrode 309, respectively. A twisted nematic liquid crystal material 312 is sandwiched between these aligning films 311.
This type of liquid crystal display apparatus is ordinarily used in a so-called "normally white mode", in which when no driving voltage is applied to the liquid crystal, the liquid crystal permits transmission of light, and when a driving voltage is applied to the liquid crystal, the liquid crystal blocks transmission of light. In this type of liquid crystal display apparatus, however, since it is not possible to control the liquid crystal in an region between the pixel electrodes 306, it is not possible to elevate the lightness/darkness ratio of an overall display screen. In addition, at a periphery of the pixel electrode 306, a direction of the electric field becomes out of perpendicular to the electrode, abnormal orientation such as disclination occurs in the liquid crystal layer, which becomes a cause of display defective such as image sticking and afterimage.
Under this circumstance, there has been ordinarily provided a light block layer in a pixel boundary area. Generally, as shown in FIG. 1B, the inter-pixel light blocking layer 310 is provided on the opposing plate 308, so as to conceal from a pixel aperture portion the pixel boundary area and its neighbor zone where the transmission light cannot be controlled. In addition, since it is known that the disclination occurs particularly in a region depending upon an initial orientation direction determined by a rubbing direction and a direction of an applied electric field, it has been proposed that the inter-pixel light blocking layer 310 is extended to cover an area in which the above mentioned orientation abnormality is apt to easily appear (Japanese Patent Publication JP-A-01-266512). Alternatively, it has been also proposed that the pixel electrode 306 is extended (Japanese Patent Publication JP-A-02-013927). With these means, the area in question is concealed in particular.
Furthermore, as shown in FIG. 2, a storage capacitor electrode 407 provided under a pixel electrode 406 is formed to extend along a periphery of the pixel electrode so as to improve a pixel aperture ratio (Japanese Patent Publication JP-A-03-239229). Incidentally, in FIG. 2, Reference Numeral 402 designates a scan line, and Reference Numeral 403 shows a signal line. Reference Numeral 405 indicates a switching device such as TFT.
As mentioned above, the orientation abnormality of the liquid crystal occurs at the periphery of the pixel electrode. In this areas, however, it is not possible to control the transmission light. Therefore, if the orientation abnormality of the liquid crystal occurs within the pixel aperture, the display is affected by the above mentioned adverse influence. Since the liquid crystal device of the normally white mode is configured to block light when a voltage is applied, it is difficult to enlarge the aperture area to the size of the pixel electrode. On the other hand, in the case that the inter-pixel light blocking layer is provided on the opposing plate, an extended margin corresponding the amount of misalignment which would occur when the pair of plates are coupled to each other as shown in FIG. 1B, becomes necessary. In this case, an angle of field in relation to the pixel electrode must be considered, and therefore, the area of the aperture must be greatly reduced in comparison with the area of the pixel electrode.
To minimize the amount of misalignment, if a high precision of alignment is required in the process of bonding the pair of plates, the manufacturing process becomes correspondingly difficult. Since the liquid crystal device incorporated in the projection type display apparatus does not require to consider the angle of field, it becomes unnecessary to lower the aperture ratio by the amount corresponding to the fact that it is not necessary to consider the angle of field. However, in order to miniaturize the whole of the plate, an interval between the pixel electrodes is shortened, so that interaction between the electrodes becomes liable to occur, with the result that the orientation abnormality of the liquid crystal easily occurs within the pixel.
If the aperture area of the pixel is reduced so as to lower the aperture ratio, the power consumption increases, or alternatively, the interval between the pixel apertures becomes effectively large. These leads to deterioration of the image quality.
Under the above mentioned circumstance, there is demanded a means capable of ensuring the aperture ratio without extremely increasing the precision of alignment in the plate bonding and without lowering the image quality.
When the inter-pixel light blocking layer is provided on the main plate, since the precision of alignment of the light blocking layer to the pixel electrode can be determined by the precision of photolithography, it is possible to easily obtain a high precision. Therefore, it becomes unnecessary to reduce the aperture while considering the misalignment of the plates and the angle of field. However, if a lateral electric field is applied between the inter-pixel light blocking layer and the pixel applied with a saturation voltage, the orientation abnormality of the liquid crystal occurs on the pixel electrode.
In the case that the light blocking layer 407 is provided under the periphery of the pixel electrode 406 in a positional relation as shown in FIG. 2, it is possible to conceal the orientation abnormality of the liquid crystal. However, in order to reduce an overlapping between the wiring electrodes 402 and 403 and the storage capacitor electrode 407, the light blocking layer is realized in the form of a closed loop so as to be positioned only the periphery of the pixel electrode. This causes the following disadvantages:
First, since a gap occurs between the wiring electrodes and the storage capacitor electrode, a light block layer for concealing the gap has to be provided on the opposing plate. In addition, the storage capacitor electrode 407 is required to be formed in the shape of a narrow width band, it is difficult to narrow the electrode because of restriction attributable to an electric resistance and a pattern forming process, and therefore, it is not possible to so enlarge the area of the aperture. Even if the electrode were narrowed, since the light blocking layer of the opposing plate can have only a margin for misalignment corresponding to the width of the storage capacitor electrode, a high precision of alignment is still required in order to increase the aperture ratio.
In the case that the light blocking is necessary because the switching device is sensitive to light, another problem occurs. In the structure shown in FIG. 2, since the wiring electrode or another on a plate opposite to a light incident plate is viewed within the aperture regardless of whether light is incident to either the main plate or the opposing plate, the incident light is reflected by the wiring electrode, so that the light passes into the switching device, with the result that the characteristics of the switching device is deteriorated.